Method for bonding heatsink to multiple-height chip

ABSTRACT

A method and structure for thermally connecting a thermal conductor to at least one chip, the thermal conductor including a lower surface and at least one piston extending from the lower surface corresponding to each of the chips, each of the chips having an upper surface opposing each of the pistons, the chips being mounted on a substrate, the method comprising steps of metalizing the lower surface of the thermal conductor and the pistons, applying a solder to the lower surface of the thermal conductor, applying a thermal paste between the upper surface of the chips and the pistons, positioning the substrate and the thermal conductor such that the substrate is aligned with the thermal conductor, biasing the thermal conductor toward the substrate, biasing the pistons toward the chips such that the thermal paste has a consistent thickness between each of the chips and the pistons, reflowing the solder, such that the solder bonds the substrate to the thermal conductor and the pistons form a metallurgical bond with the thermal conductor, wherein after the reflowing step, the pistons and the thermal conductor form a unitary structure for maintaining the consistent thickness of the thermal paste between each of the chips and the pistons which achieves a considerably thinner thermal paste layer and greater thermal conduction.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to devices and methodsfor thermally connecting heatsinks and other heat conductive caps tosemiconductor chips.

[0003] 2. Description of the Related Art

[0004] Conventional systems utilize adhesives and thermally conductivepastes to connect cap/integral heatsinks to semiconductor substrate/chipstructures. For example, in conventional structures, a layer of thermalpaste is placed between the chip and the cap/integral heatsink and thecap/integral heatsink is then joined to the substrate.

[0005] However, the height of the chip above the substrate could varyfrom chip to chip. This is especially true with multi-chip packages. Inthe case of a paste interface, when cap/integral heatsink dimensionaltolerances are included, the thickness of the thermal paste between thetop of the chip and the bottom of the cap/integral heatsink could alsovary by as much as ±3 mils.

[0006] Since the thickness of the thermal paste in conventionalstructures could vary by ±3 mils (and generally exceeds 6 mils innominal thickness) the heat transfer characteristics of conventionalstructures vary widely and are less efficient than if the thermal pastegap could be formed more repeatedly and thinner. A discussion of someconventional structures follows.

[0007] Some conventional structures utilize a complex cooling hat. Overeach of the flip chips on the substrate is a hole in the hat. From eachhole extends a spring loaded piston that contacts the back of the chip.These modules are hermetic, and are filled with helium. The primarycooling path is from the circuit side of the chip, through the thicknessof the chip, to the face of the piston, up the piston, to the inside ofthe hat, to the back of the hat, across an interface, to an attachedcold plate and to water circulating through the cold plate. The highhelium content of the gas in the module greatly reduces the thermalresistances of the chip-to-piston interface and the piston-to-hatinterface.

[0008] Conventional structures also include a matrix of pistons thatcontact the back of the chip for cooling. To maintain almost fullcoverage of the back of the chip, headers are used on the faces of eachof the pistons.

[0009] Conventional structures also use barrel shaped pistons to allowtighter piston to hat gaps, while maintaining the ability to accommodatechip tilt. Material changes also improve thermal performance. Forexample, conventionally pistons are made of copper rather than aluminum,and the module can be filled with oil rather than helium.

[0010] Further, conventionally solder is included with each of thepistons so that the solder can be reflowed after assembly, to fill thechip-to-piston and piston-to-hat gaps, for improved thermal performance.

[0011] In conventional flat plate cooling (FPC), a flat plate just abovethe array of chips is water cooled. Thermal paste is used to fill thegaps between the chips and the flat plate.

[0012] Stable high solids, high thermal conductivity paste is alsoconventionally available. High thermal conductivity is accomplished byhigh solids loading, which is accomplished by using a range of particlesizes, and coating the particles with a dispersant. This allows asignificant improvement in the thermal conductivity of thermal pastes.

[0013] Other conventional structures use spreader plates between thechips and the hat. The spreader plates are soldered to the backs of thechips and then thermally connected to the inside of the hat by a layerof thermal paste.

[0014] Conventional structures also include a thermal path that leadsfirst to a cooling plate which has holes that house pistons that will belocked into position. The pistons are joined to the side-walls of theholes by solder, and the piston faces contact the backs of the chips.Conventionally, thermal paste is used to fill the chip-to-pistoninterface. The assembly conventionally requires two steps, a set up stepto reflow the solder and lock the pistons into their final positions(customized to that module) and a separate assembly step where the hatis attached to the substrate, enclosing the chips. The mating surfacesof the hat and piston are metalized for solderability.

[0015] The conventional structures use water circulating systemsapplicable to high-end thermal conduction modules (TCM's), which featurean interface with the external plate which curtails thermal conductionand which is restricted to limited choices of materials, while thedesign is not readily extendible to air cooled multichip modules (MCM)and low-end MLC modules.

SUMMARY OF THE INVENTION

[0016] It is, therefore, a purpose of the present invention to provide astructure and method for attaching cap/integral heatsinks tosemiconductor chips and more specifically to forming a consistently thinlayer of a thermal paste between the cap/integral heatsink and thechips.

[0017] More specifically, the invention includes a method for thermallyconnecting a cap/integral heatsink to at least one chip, thecap/integral heatsink including a lower surface and at least one pistonextending from the lower surface corresponding to each chip, each chiphaving an upper surface opposing each piston, the chips being mounted ona substrate, the method comprising steps of metalizing the lower surfaceof the cap/integral heatsink and the pistons, applying a solder to thelower surface of the cap/integral heatsink, applying a thermal pastebetween the upper surface of the chips and the pistons, positioning thesubstrate and the cap/integral heatsink such that the substrate isaligned with the cap/integral heatsink, biasing the cap/integralheatsink toward the substrate, biasing the pistons toward the chips,such that the thermal paste has a consistent thickness between each ofthe chip and the pistons, reflowing the solder, such that the solderbonds the substrate to the cap/integral heatsink and bonds the pistonsto the cap/integral heatsink, wherein after the reflowing step, thepistons and the cap/integral heatsink form a unitary structure whichmaintains the thermal paste gap between chips and the pistons.

[0018] The metalizing step comprises metalizing the lower surface of thecap/integral heatsink and the pistons with solder wettable metallurgy.The step of applying the solder to the lower surface of the cap/integralheatsink comprises a step of applying solder preforms to areas of thelower surface of the cap/integral heatsink adjacent the pistons. Thestep of biasing the pistons toward the chips comprises a step ofinserting springs between the cap/integral heatsink and the pistons andapplying/dispensing thermal paste to chips. The step of biasing thepistons toward the at least one chip comprises a step of supplyingsufficient force between the pistons and the chips to achieve a thermalpaste gap of 3 mils. During the reflowing step, the solder fills allgaps between the pistons and the cap/integral heatsink. Each of the atleast one chip may have a different height above the substrate whencompared to others of the at least one chip and the step of biasing theat least one piston toward the at least one chip accommodates for thedifferent height.

[0019] The invention also includes a method for attaching a cap/integralheatsink to a multi-chip structure, the cap/integral heatsink includinga plurality of movable pistons opposing each of a plurality of chips ofthe multi-chip structure, the method comprising steps of applying athermal paste between each of the chips and the pistons, adjusting aposition of the pistons to form a consistent layer of thermal pastebetween the each of chips and the pistons, forming a metallurgical bondbetween the pistons and the cap/integral heatsink such that the pistonsare permanently fixed in a position to maintain the consistent layer ofthermal paste, bonding the cap/integral heatsink to the multi-chipstructure.

[0020] The step of forming a metallurgical bond comprises steps ofmetalizing a lower surface of the cap/integral heatsink and the pistons,applying a solder to the lower surface of the cap/integral heatsink,assembling the heat sink and the multi-chip structure and reflowing thesolder such that the solder forms the metallurgical bond. The metalizingstep comprises metalizing the lower surface of the cap/integral heatsinkand the pistons. The step of applying the solder comprises a step ofapplying solder preforms. The step of applying the thermal pastecomprises applying the thermal paste to the backs of the chips byscreening or use of an automated dispense tool. The step of adjusting aposition of the pistons comprises a step of inserting springs betweenthe cap/integral heatsink and the pistons. The step of adjusting aposition of the pistons comprises a step of supplying sufficient forcebetween the pistons and the chips to consistently narrow the layer ofthe thermal paste to about 3 mils. The chips may have different heightsand the step of adjusting the position of the pistons accommodates forthe different heights.

[0021] The invention also includes a method for attaching a cap/integralheatsink to a multi-chip structure having a substrate and a plurality ofchips mounted on the substrate, the cap/integral heatsink including alower surface and a plurality of pistons extending from the lowersurface corresponding to each of the chips, each of the chips having anupper surface opposing a corresponding one of the pistons, the methodcomprising steps of metalizing the lower surface of the cap/integralheatsink and the at least one pistons with solder wettable metallurgy,applying a plurality of solder preforms to the lower surface of thecap/integral heatsink adjacent each of the pistons, applying a thermalpaste comprising a conductive solid particle dispersion/suspensionbetween the upper surface of each of the chips and the pistons,positioning the substrate and the cap/integral heatsink such that thesubstrate is aligned with the cap/integral heatsink, biasing thecap/integral heatsink toward the substrate with a clamp, biasing thepistons toward the chips with springs, such that the thermal paste has aconsistent thickness between each of the chips and pistons, reflowingthe solder preforms, such that the solder bonds the substrate to thecap/integral heatsink and reacts with the solder wettable metallurgy, toform a metallurgical bond between the pistons and the cap/integralheatsink and fills all gaps between the pistons and the cap/integralheatsink, wherein after the reflowing step, the pistons and thecap/integral heatsink form a unitary structure which maintains theconsistent thickness of the thermal paste between each of the chips andthe pistons.

[0022] The invention also includes a multi-chip structure comprising asubstrate, a plurality of chips mounted on the substrate, a cap/integralheatsink mounted on the substrate and covering the chips, thecap/integral heatsink including a plurality of fixed pistons, each ofthe pistons extending from the cap/integral heatsink toward acorresponding chip of the chips, and a thermal paste positioned betweenthe pistons and the chips, wherein each of the pistons extends from thecap/integral heatsink a unique distance such that a distance betweeneach piston and the corresponding chip comprises a consistent distanceand the thermal paste has a consistent thickness between all the pistonsand the chips.

[0023] The thermal paste comprises a conductive solid particledispersion. The cap/integral heatsink is bonded to the substrate withone of a seal material comprising solder and a polymer. The pistons aremetallurgically bonded to the cap/integral heatsink through solderreaction with the metalization layers. The chips may have differentheights above the substrate, and the pistons extending from thecap/integral heatsink have a unique distance which accommodates for thedifferent chip heights.

[0024] The invention also includes a multi-chip structure including asubstrate having a plurality of chips and a cap/integral heatsink, thecap/integral heatsink including a lower surface and a plurality ofpistons extending from the lower surface corresponding to each of thechips, each of the chips having an upper surface opposing acorresponding one of the pistons, the multi-chip structure being formedby a process comprising steps of metalizing the lower surface of thecap/integral heatsink and the pistons, applying a solder to the lowersurface of the cap/integral heatsink, applying a thermal paste betweenthe upper surface of the chips and the corresponding one of the pistons,positioning the substrate and the cap/integral heatsink such that thesubstrate is aligned with the cap/integral heatsink, biasing thecap/integral heatsink toward the substrate, biasing the pistons towardthe chips, such that the thermal paste has a consistent thicknessbetween each of the chips and the pistons, reflowing the solder, suchthat the solder bonds the substrate to the cap/integral heatsink and thepistons form a metallurgical bond with the cap/integral heatsink,wherein after the reflowing step, the pistons and the cap/integralheatsink form a unitary structure which maintains the consistentthickness of the thermal paste between the each of the chips and thecorresponding one of the pistons.

[0025] The step of applying the thermal paste comprises applying thethermal paste to the backs of the chips by screening or use of anautomated dispense tool. The metalizing step comprises metalizing thelower surface of the cap/integral heatsink and the pistons. The step ofapplying the solder to the lower surface of the cap/integral heatsinkcomprises a step of applying solder preforms to areas of the lowersurface of the cap/integral heatsink adjacent the pistons. The step ofbiasing the cap/integral heatsink toward the substrate comprises a stepof temporarily clamping the cap/integral heatsink to the substrate. Thestep of biasing the pistons toward the chips comprises a step ofinserting springs between the cap/integral heatsink and the pistons. Thestep of biasing the pistons toward the chips comprises a step ofsupplying sufficient force between the pistons and the chips to narrowthe consistent thickness of the thermal paste to about 3 mils. Duringthe reflowing step, the solder fills all gaps between the pistons andthe cap/integral heatsink. Each of the chips may have a different heightabove the substrate when compared to others of the chips and the step ofbiasing the pistons toward the chips accommodates for the differentheights.

[0026] With the invention, the thermal paste is maintained at athickness of about 3 mils between the top of the semiconductor chip andthe bottom of the cap/integral heatsink, regardless of variation in chipof height. Therefore, with the invention, superior and consistentthermal transfer characteristics are produced when compared toconventional structures which have a thicker and more variable layer ofthermal paste.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The foregoing and other objects, aspects and advantages will bebetter understood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

[0028]FIG. 1 is a schematic diagram of a chip/substrate structure andheat sink structure; and

[0029]FIG. 2 is a flow diagram illustrating a preferred method of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0030] Referring now to the drawings, and more particularly to FIG. 1, acap/integral heatsink 1 is illustrated. The cap/integral heatsink 1includes cylinders, blind holes or piston holes 8 which hold pistons 2which are biased toward the chips 4 via springs 3. One or moresemiconductor chips 4 are thermally connected to the cap/integralheatsink 1. The chips 4 are mounted on a substrate 5 with solder balls 6or other conventional attachment structures. A thermal paste 7 or othercompliant thermal conductor material connects the pistons 2 to the topof the chips 4.

[0031] While the structure illustrated in FIG. 1 is a cap or an integralheatsink with cooling fins (referred to herein as “cap/integralheatsink”), as would be known by one ordinarily skilled in the art giventhis disclosure, the invention is equally applicable to any thermalconductor which is mounted over chips (which vary in height) where aconsistent thickness of thermal paste between the top of the chips andthe thermal conductor is important. The invention is not limited to theembodiment of the invention shown in FIG. 1 (e.g., a cap/integralheatsink), but instead is applicable to any similar heat dissipatingstructure (e.g., air cooled, liquid cooled, etc.) which is thermallyconnected to heat generating chips.

[0032] The structure shown in FIG. 1 is a multi-chip package (such as a4×4 chip arrangement). However, the invention is suitable for any typeof chip package which requires heat dissipation including single chipstructures. Additionally, the invention is applicable to any type ofstructure which covers a variety of elements with a single cap/integralheatsink.

[0033] The composition of the individual elements shown in FIG. 1 iswell known to those ordinarily skilled in the art and could be formedfrom a wide selection of materials and substances. For example, thecap/integral heatsink 1 could be formed of many materials such asaluminum, aluminum nitride, silicon carbide, aluminum silicon carbide,tungsten copper, etc. The pistons 2 could be formed of aluminum,aluminum nitride, silicon carbide, aluminum silicon carbide, tungstencopper or other similar conductive materials that match the thermalcoefficient of expansion of the cap/integral heatsink. Similarly, thesubstrate 5 could be formed from many materials such as alumina, silica,glass, glass ceramic, ceramic, mullite, aluminum nitride or other wellknown materials. The semiconductor chips 4 could be attached to thesubstrate 5 with a number of well known bonding agents such as solder.Also, the thermal paste 7 could be a conductive solid particledispersion or any similarly suitable compliant thermally conductivematerial.

[0034] Such a chip/substrate structure commonly exhibits largevariations in the chip heights. More specifically, a difference in chipheights on different modules could vary by several mils. This variationin chip heights presents a thermal variation in conventional structureswhich attach a flat (e.g., machined) cap/integral heatsink acrossmultiple chips 4 because the difference in chip height will causedifferent thicknesses of thermal paste 7 to form in the gaps between thetop of the chips 4 and the bottom of the cap/integral heatsink, 1.

[0035] The invention overcomes the problems of conventional structuresby utilizing a unique method and structure to thermally connect thecap/integral heatsink 1 to the chip 4. More specifically, the bottom ofthe cap/integral heatsink 1 (including the cylinders 8 and pistons 2) ismetalized with at least one layer of solder wettable material by commondeposition methods, such as sputtering, deposition and evaporation toform a solder wettable metalization layer 9. The side walls of thepistons are also metalized.

[0036] A seal 11 is formed between the substrate 5 and the cap/integralheatsink 1 to attach the cap/integral heatsink 1 firmly to the substrate5. Such seal could also comprise any suitable solder or polymer.

[0037] The substrate 5 is aligned with and biased against thecap/integral heatsink 1 either by clamp or gravity pressure (e.g.,additional weight). The solders are heated to a sufficient temperatureto cause the solders 10, 11 to melt (e.g., to reflow) and the springsbias the pistons 2 toward the chips 4. If the structure is sealed with apolymer rather than solder, sufficient time at elevated temperature isneeded to cure the adhesive.

[0038] The spring constants are selected such that sufficient pressureis generated against the thermal paste 7 so as to squeeze the thermalpaste 7 to a consistent thickness of about 5 mils, 3 mils or lessbetween the pistons and the top of the chip 4 before or during thereflow process.

[0039] The solder 10 flows between the pistons 2 and cylinder 8 andreacts with the metalization layer 9. The solder 10 flows around thepistons 2 by wetting, capillary and gravitational actions. Thus, whenthe structure cools below the melting point of the solder 10 (e.g.,after the reflow process), there are no gaps between the pistons 2 andthe surface of the cap/integral heatsink 1. Additionally, after thereflow process, the cap/integral heatsink 1 is firmly attached to thesubstrate 5 by cured adhesive or solder joints 11.

[0040] Further, after the solder 10 has reacted with the metalizationlayer 9, the pistons 2 become metallurgically connected to the cylinder8 of the cap/integral heatsink 1. After the reflow process, the pistons2 are firmly held in place and cannot move within the cylinders or blindholes 8.

[0041] In order to facilitate there being no gaps in the solder 10within the cylinders 8 of the cap/integral heatsink 1 (e.g., around thepistons 2), a vent hole (not shown) may be made in the pistons 2 and/ortop of the cap/integral heatsink 1 to allow any gasses within thecylinder 8 to escape during the reflow and cooling process. Therefore,after the reflow process, the cap/integral heatsink 1 and pistons 2 forma unitary structure which has a higher thermal performance than astructure with movable pistons. Further, as a result of the reflowprocess and the action of the springs 3, the unitary cap/integralheatsink 1 structure is customized for the different chip heights of thechip matrix.

[0042] By using the springs 3 to maintain constant and controlledpressure between the pistons 2 of the cap/integral heatsink 1 and thechip 4 during the reflow process, the thickness of the thermal materialbetween the cap/integral heatsink 1 and the chip 4 is strictlycontrolled. More specifically, with the above structure and method, thethickness of the thermal material can be consistently formed to anydesired thickness (e.g., about 3 mils) for each chip 4 regardless ofvariation of the height of chips 4 on a single substrate 5.

[0043] The metallurgical connection between the pistons 2 and thecap/integral heatsink 1 provides an extremely low thermal resistance.Additionally, the 3 mil thickness of the thermal paste 7 reduces thethermal resistance when compared to conventional structures which havethicker layers of thermal paste. Indeed, the internal thermal resistanceof the inventive structure is reduced by a factor of two when comparedto conventional structures.

[0044] Additionally, the inventive structure is superior to structureswhich utilize springs to maintain pressure between movable pistons andthe top of the chips 4 because such free moving pistons do not have ametallurgical bond to the cap/integral heatsink and such a conventionalcap/integral heatsink does not have a unitary structure. Thus, thethermal conductivity of such a conventional structure is dramaticallyless than the unitary structure of the present invention.

[0045] Referring now to FIG. 2, a preferred method of the invention isillustrated. More specifically, in block 21, the pistons 4 and thebottom of the cap/integral heatsink 1 are metalized. In block 22,springs and pistons are inserted into the cylinders while the structureis inverted. In block 23, the solder preforms 10 are applied to thecap/integral heatsink 1. In block 24, a thermal paste is applied to thechip 4. In block 25, the cap/integral heatsink 1 is temporarily attachedto the substrate 5 (i.e., by clamping) and the pistons 2 are biased(e.g., by the springs 3) to form a consistently thin layer of thermalpaste 7 between the pistons 2 and the chip 4. In block 26 the solder isreflowed to metallurgically bond the pistons 2 to the cap/integralheatsink 1 and to completely fill the gap between the pistons and thecap/integral heatsink.

[0046] The cap/integral heatsink is also bonded to the substrate in thesame reflow operation. By accommodating the different heights of variouschips 4 connected to a substrate 5, through the use of moveable pistons2 and springs, and firmly forming a unitary structure permanentlyconnected to the substrate 5, the inventive method and structure takesadvantage of the increased thermal characteristics of a unitarystructure as well as the benefits which are received through the use ofone-time adjustable pistons 2.

[0047] With the invention, the thermal conductive capacity is increasedwithout expensive refrigeration or the use of exotic liquid coolingmaterials. Further, the metallurgical connection between the pistons andthe cap/integral heatsink 1 insures that the thermal capacity of theinventive structure will have a useful lifetime.

[0048] The inventive method and structure is effective for single chipapplications, multiple chip applications and multi-chip packages, suchas a 4×4 chip arrangement.

[0049] The present invention is generally applicable to the entire gamutof modules from large multi-chip modules (MCM's) to the smallsingle-chip modules (SCM's). The invention includes a unitary cap orintegral heat-sink that reduces the internal thermal resistanceconsiderably by virtue of a reduced thickness thermal paste filledchip-to-piston interface. The thickness of this paste filled gap isachieved and tightly controlled by use of precision standoffs 12 in thegap. Standoffs are small (3 mils thick) pads 12 and prevent the pistonfrom contacting the chip surface and, therefore, further control thethickness of the thermal paste filled gap. The standoffs may be attachedto the pistons, placed on the chip, or preloaded into the thermal paste.The spring against each piston has sufficient force to squeeze thethermal paste out to the thickness of the standoffs 12 (e.g, 3 mils),before or during the reflow step.

[0050] The invention utilizes a novel process of soldering the pistonsto the walls of the holes in the cap/integral heat sink using preformsand capillary action during solder reflow. The pistons are thermallyconnected to the chips by thermal paste. Pumping of the paste due topower cycling, and stack up tolerances, limit the practical minimumthickness of the thermal paste in conventional designs. Since this novelapproach overcomes stack-up tolerance drawbacks, thinner gaps may beachieved, improving thermal performance. Nominal gaps may be reducedfrom 6 mils to 3 mils, reducing internal thermal resistance by a factorof two.

[0051] One aspect of the current invention is to reduce and control thethickness of the thermal paste filled gap, to improve thermalperformance. Another aspect of the current invention is that the sealingof the cap/integral heat sink to the substrate, the soldering of thepistons, and coupling of the pistons with the chip is donesimultaneously.

[0052] The invention is applicable to narrower gaps (e.g., 2 mils, 1.5mils) also. The chip-to-piston gap thickness is very thin and tightlycontrolled.

[0053] Conventional structures which dissipate heat in high-endwater-cooled modules are rather expensive (or inadequate) when used withair cooled modules. The current invention opens a new way to obtainefficient cooling across various technologies in a unique way, in termsof unitary cap/integral heat sink, novel controlled paste process andnovel solder preform applications. The invention has the versatility touse materials having an optimal match of thermomechanical properties,resulting in more efficient thermal enhancement system.

[0054] Therefore, with the invention the thermal characteristics of thecap/integral heatsink are improved over the conventional structures.

[0055] The invention allows higher power chips to be cooled. Theinvention enhances the performance of cap/integral heatsinks because itallows narrower thermal paste filled gaps between chips and cap/integralheatsinks. The narrower gaps that are achieved are reliable because ofthe large reduction in gap tolerance.

[0056] While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

What is claimed is:
 1. A method for thermally connecting a thermalconductor to at least one chip, said thermal conductor including a lowersurface and at least one piston extending from said lower surface, eachof said at least one chip having an upper surface opposing each of saidat least one piston, said at least one chip being mounted on asubstrate, said method comprising steps of: metalizing said lowersurface of said thermal conductor and said at least one piston; applyinga solder to said lower surface of said thermal conductor; applying athermal paste to said upper surface of said at least one chip;positioning said substrate and said thermal conductor such that saidsubstrate is aligned with said thermal conductor; biasing said thermalconductor toward said substrate; biasing said at least one piston towardsaid at least one chip, such that said thermal paste has a consistentthickness between each of said at least one chip and said at least onepiston; and reflowing said solder, such that said solder bonds saidsubstrate to said thermal conductor and said at least one piston forms ametallurgical bond with said thermal conductor, wherein after saidreflowing step, said at least one piston and said thermal conductor forma unitary structure for maintaining said consistent thickness of saidthermal paste between each of said at least one chip and said at leastone piston.
 2. The method as in claim 1, wherein said metalizing stepcomprises a step of metalizing said lower surface of said thermalconductor and said at least one piston with a solder wettablemetalization.
 3. The method as in claim 1, wherein said step of applyingsaid solder comprises a step of applying solder preforms to areas ofsaid lower surface of said thermal conductor adjacent said at least onepiston.
 4. The method as in claim 1, wherein said step of biasing saidthermal conductor toward said substrate comprises a step of temporarilyclamping said thermal conductor to said substrate.
 5. The method as inclaim 1, wherein said step of biasing said at least one piston towardsaid at least one chip comprises a step of inserting at least one springbetween said thermal conductor and said at least one piston.
 6. Themethod as in claim 1, wherein said step of biasing said at least onepiston toward said at least one chip comprises a step of supplyingsufficient force between said at least one piston and said at least onechip to narrow said consistent thickness of said thermal paste to lessthan 5 mils.
 7. The method as in claim 1, wherein said step of biasingsaid at least one piston toward said at least one chip comprises a stepof supplying sufficient force between said at least one piston and saidat least one chip to narrow said consistent thickness of said thermalpaste to about 3 mils.
 8. The method as in claim 1, wherein said step ofbiasing said at least one piston toward said at least one chip comprisesa step of supplying sufficient force between said at least one pistonand said at least one chip to narrow said consistent thickness of saidthermal paste to less than 3 mils.
 9. The method as in claim 1, whereinduring said reflowing step, said solder fills all gaps between said atleast one piston and said thermal conductor.
 10. The method as in claim1, wherein each of said at least one chip has a different height abovesaid substrate and said step of biasing said at least one piston towardsaid at least one chip accommodates for said different height.
 11. Amethod for thermally connecting a thermal conductor to a multi-chipstructure, said thermal conductor including a plurality of movablepistons opposing each of a plurality of chips of said multi-chipstructure, said method comprising steps of: applying a thermal pastebetween each of said chips and said pistons; adjusting a position ofsaid pistons such that said thermal paste has a consistent thicknessbetween each of said chips and said pistons; forming a metallurgicalbond between said pistons and said thermal conductor such that saidpistons are permanently fixed in a position to maintain said consistentthickness; and bonding said thermal conductor to said multi-chipstructure.
 12. The method as in claim 11, wherein said step of forming ametallurgical bond comprises steps of: metalizing a lower surface ofsaid thermal conductor and said pistons; applying a solder to said lowersurface of said thermal conductor; assembling said thermal conductor andsaid multi-chip structure; and reflowing said solder such that saidsolder forms said metallurgical bond.
 13. The method as in claim 12,wherein said metalizing step comprises a step of metalizing said lowersurface of said thermal conductor and said pistons with solder wettablemetallurgy.
 14. The method as in claim 12, wherein said step of applyingsaid solder comprises a step of applying solder preforms.
 15. The methodas in claim 11, wherein said step of adjusting a position of saidpistons comprises a step of inserting springs between said thermalconductor and said pistons.
 16. The method as in claim 11, wherein saidstep of adjusting a position of said pistons comprises a step ofapplying a force between said pistons and said chips to narrow saidconsistent thickness of said thermal paste to less than 5 mils.
 17. Themethod as in claim 11, wherein said step of adjusting a position of saidpistons comprises a step of applying a force between said pistons andsaid chips to narrow said consistent thickness of said thermal paste toabout 3 mils.
 18. The method as in claim 11, wherein said step ofadjusting a position of said pistons comprises a step of applying aforce between said pistons and said chips to narrow said consistentthickness of said thermal paste to less than 3 mils.
 19. The method asin claim 11, wherein each of said chips has a different height and saidstep of adjusting said position of said pistons accommodates for saiddifferent height.
 20. A method for thermally connecting a thermalconductor to a multi-chip structure having a substrate and a pluralityof chips mounted on said substrate, said thermal conductor including alower surface and a plurality of pistons extending from said lowersurface, each of said chips having an upper surface opposing acorresponding one of said pistons, said method comprising steps of:metalizing said lower surface of said thermal conductor and said pistonswith solder wettable metallurgy; applying a plurality of solder preformsto said lower surface areas of said thermal conductor adjacent each ofsaid pistons; applying a thermal paste to said upper surface of saidchips;; positioning said substrate and said thermal conductor such thatsaid substrate is aligned with said thermal conductor; clamping saidthermal conductor against said substrate; biasing said pistons towardsaid chips with springs, such that said thermal paste has a consistentthickness between each of said chips and pistons; and reflowing saidsolder preforms, such that said solder bonds said substrate to saidthermal conductor, reacts with said solder wettable metalization to forma metallurgical bond between said pistons and said thermal conductor,and fills all gaps between said pistons and said thermal conductor,wherein after said reflowing step, said pistons and said thermalconductor form a unitary structure for maintaining said consistentthickness of said thermal paste between each of said chips and saidpistons.
 21. A multi-chip structure comprising: a substrate; a pluralityof chips mounted on said substrate; a thermal conductor mounted on saidsubstrate and covering said chips, said thermal conductor including aplurality of fixed pistons, each of said pistons extending from saidthermal conductor toward a corresponding chip of said chips; and athermal paste positioned between said pistons and said chips, whereineach of said pistons extends from said thermal conductor a uniquedistance such that a distance between each of said pistons and saidcorresponding chip comprises a consistent distance and said thermalpaste has a consistent thickness between all of said pistons and saidchips.
 22. The multi-chip structure in claim 21, wherein said thermalconductor is bonded to said substrate with a seal comprising one ofsolder and a polymer.
 23. The multi-chip structure in claim 21, whereinsaid pistons are metallurgically bonded to said thermal conductor withthe metalization layers reacted with solder.
 24. The multi-chipstructure in claim 17, wherein each of said chips has a different heightabove said substrate, said pistons extending from said thermal conductorsaid unique distance accommodates for said different height.
 25. Amulti-chip structure including a substrate having a plurality of chipsand a thermal conductor, said thermal conductor including a lowersurface and a plurality of pistons extending from said lower surface,each of said chips having an upper surface opposing a corresponding oneof said pistons, said multi-chip structure being formed by a processcomprising steps of: metalizing said lower surface of said thermalconductor and said pistons; applying a solder to said lower surface ofsaid thermal conductor; applying a thermal paste to said upper surfaceof said chips;; positioning said substrate and said thermal conductorsuch that said substrate is aligned with said thermal conductor; biasingsaid thermal conductor toward said substrate; biasing said pistonstoward said chips, such that said thermal paste has a consistentthickness between each of said chips and said pistons; and reflowingsaid solder, such that said solder bonds said substrate to said thermalconductor and said pistons form a metallurgical bond with said thermalconductor, wherein after said reflowing step, said pistons and saidthermal conductor form a unitary structure for maintaining saidconsistent thickness of said thermal paste between each of said chipsand each of said corresponding pistons.
 26. The multi-chip structure asin claim 25, wherein said metalizing step comprises metalizing saidlower surface of said thermal conductor and said pistons with solderwettable metallurgy.
 27. The multi-chip structure as in claim 25,wherein said step of applying said solder to said lower surface of saidthermal conductor comprises a step of applying solder preforms to areasof said lower surface of said thermal conductor adjacent said pistons.28. The multi-chip structure as in claim 25, wherein said step ofbiasing said thermal conductor toward said substrate comprises a step oftemporarily clamping said thermal conductor to said substrate.
 29. Themulti-chip structure as in claim 25, wherein said step of biasing saidpistons toward said chips comprises a step of inserting springs betweensaid thermal conductor and said pistons.
 30. The multi-chip structure asin claim 25, wherein said step of biasing said pistons toward said chipscomprises a step of applying a force between said pistons and said chipsto narrow said consistent thickness of said thermal paste to about 3mils.
 31. The multi-chip structure as in claim 25, wherein during saidreflowing step, said solder fills all gaps between said pistons and saidthermal conductor.
 32. The multi-chip structure as in claim 25, whereineach of said chips has a different height above said substrate and saidstep of biasing said pistons toward said chips accommodates for saiddifferent height.
 33. The multi-chip structure as in claim 25, furthercomprising at least one standoff positioned between at least one of saidthe chips and said corresponding one of said pistons.